The present invention concerns a neck block of a moulding tool for the production of a hollow body preform by means of injection moulding, wherein the neck block has a main body with a shaping surface which is adapted to the external contour of a neck portion of the hollow body preform to be produced and is provided to come into contact in the injection moulding operation with the neck portion of the hollow body preform. In injection moulding the material is plasticised and then injected under high pressure into an injection moulding tool. The material remains in the tool until it goes into a solid condition in which it can be removed from the mould. For that purpose the injection moulding tool forms a substantially closed chamber, the contour of which corresponds to the intended external contour of the casting to be produced.
In the production of PET bottles, firstly suitable hollow body preforms are produced by means of injection moulding, which are only then blown to give the definitive bottle shape in a further method step. The hollow body preforms produced by means of injection moulding for PET bottles are also referred to as blanks. The preform to be produced in the injection moulding tool has a screw thread in the region of the subsequent bottleneck and generally a neck ring. Both the screw thread and also the neck ring form undercut configurations in the shaping tool so that the injection moulding tool must be of a multi-part structure at least in that region to permit the preform to be removed from the mould. The injection moulding tool portion forming the external contour of the bottle screw thread and the neck ring is generally of a two-part structure, wherein each of the two parts is referred to as a neck block and each part has a 180° peripheral portion of the screw thread.
In general it is desirable in regard to injection moulding for the corresponding wall thicknesses of the preforms to be reduced. Besides the saving on material and weight that has the additional advantage that the cycle time, that is to say the time over which the preform is disposed in the mould cavity of the tool in order to go into a condition in which the preform is removable from the tool without deformation phenomena is reduced. Preforms of smaller wall thickness more quickly adopt a condition in which they are stable in respect of shape.
In the meantime the development towards ever thinner wall thicknesses has progressed to such a degree that, in regard to preforms the region of the screw thread and the neck ring is of the greatest wall thickness and cooling of that region of the preform is the factor which determines the cycle time. Because of the formation of the screw thread, an increased waiting time is required until the plasticised material has everywhere gone into a removable condition, even in the region of the screw thread and the neck ring respectively.
To shorten the cycle times when injection moulding preforms, it is therefore already usual for two interconnected cooling bores to be introduced into the neck blocks, through which bores a cooling fluid, mostly water, is supplied and discharged. For that purpose the cooling bores are of a substantially V-shaped configuration. That is to say, firstly a blind cooling bore is formed in the injection moulding tool portion and then a second cooling bore with a bore axis which is inclined relative to the first bore axis is formed, wherein the two bores meet in the tool body so that cooling fluid can be introduced into the tool body through the one cooling bore and then flows through the first cooling bore into the second one and can again leave the tool body by way of the second cooling bore. In these structures however the cooling agent can only be introduced in highly point relationship. That results only in low levels of cooling efficiency in the neck blocks and also gives rise to irregular dissipation of heat. Furthermore non-homogenous cooling of the preform also gives rise to problems in regard to stability in respect of shape of the preform itself.
Accordingly, additional bores are already formed individually in the tool portion, the bores also terminating in the cooling bores or in the immediate proximity thereof, in which case what are referred to as cooling pins are fitted into those additional bores, the pins being made for example from silver and involving a thermal conductivity which is increased in relation to the material of the tool so that the heat can also be transported by way of the cooling pins to the cooling passage.
Apart from the fact that the provision of cooling pins leads to increased costs in manufacture of the injection moulding tool, the distribution of heat within the tool is also not optimum in those structures.
U.S. Pat. No. 5,930,882 discloses a cooling passage for a neck block, wherein the neck block is of a two-part nature. It has a radially outwardly disposed, machined portion and a radially inwardly disposed cast portion forming the shaping surface. In that arrangement, recesses are provided as cooling passages at the outer periphery of the cast portion, that is to say at its side remote from the shaping surface. They are sealed off in relation to the ambient atmosphere with the radially outwardly disposed portion of the neck block, while the feed and the discharge for the cooling fluid is also arranged in the radially outward portion of the neck block. In that case however it is found to suffer from the disadvantage that the radially inwardly disposed portion is in the form of a cast portion as, for the manufacture of neck blocks for different forms of preforms, a respective original or master mould is required for casting the cast portion. Casting moulds of that kind however are expensive to manufacture. In addition, the two-part nature of the main body of the neck block has a detrimental effect on the stability of the neck block. Fractures along the connection between the radially inwardly and the radially outwardly disposed portions are probable.